Laser-based and LED-based video projectors have been used extensively in business environments and have recently come into wide use in large-screen projection systems in home theaters. The miniaturization of projection systems has led to the development of “pico-projectors” that may be embedded in other systems, such as mobile phones and heads-up displays for vehicle dashboards, or may be implemented as stand-alone devices, such as pocket or ultra-mobile projectors that maybe be powered from a battery or an external power source.
One example of a pico-projector system is the PicoP™ projector engine developed by Microvision, Inc. The PicoP engine includes RGB laser sources, a micro-electro-mechanical system (MEMS) scanning mirror, optics and video processing electronics for receiving video data from a source and generating an image to be projected onto any viewing surface (e.g., a screen, a wall, a sheet of paper or a chair back). However, pico-projection systems such as this that use a MEMS scanning mirror face a number of technical problems that are not as critical in larger projection systems.
A conventional MEMS scanning mirror implemented in a pico-projection system is a two-dimensional scanning mirror that sweeps laser beams across a viewing surface similar to the vertical and horizontal sweep of an electron beam in a CRT-based television or monitor. The horizontal sweep is typically done at one of the resonant mode frequencies of the scanning mirror that is on the order of 18 kHz. Operating on a resonant mode allows maximum beam deflection with minimal input energy. Although the horizontal movement is sinusoidal, the image may be pre-warped by an image processor in order to compensate for the sinusoidal movement. The vertical sweep is generally desired to be an ideal saw tooth to provide a linear sweep movement from top-to-bottom with minimal retrace time, thus maximizing the allowable active video time.
Ideally, the MEMS scanning mirror would have only one resonant mode at the horizontal sweep frequency. However, in reality, the mirror has multiple resonant modes other than the horizontal sweep frequency. This complicates any approach to finding MEMS resonant modes since it is important to operate on the intended mode rather than an adjacent one.
Peak search hardware and algorithms may be employed to find the appropriate resonant mode for operating the MEMS scanning mirror. Typical peak searches require knowing signal magnitude, which requires an analog-to-digital converter (ADC). Since the sensor signal size will be low when operating at a frequency far from the resonant mode, the resolution of the ADC needs to be high enough such that a change in frequency will result in a change of at least one ADC count. Otherwise, no direction information is available. In addition, peak search algorithms do not know inherently which way to move. Therefore, two measurements are required for every move to determine in which direction to move. Finally, peak searches are susceptible to local minima/maxima, which can trap the search at a suboptimum point. Typically, peak search algorithms overcome this difficulty by making some search inquiries far away from the current operating point. However, this complicates the algorithm and forces less than optimal operation for some period of time, which reduces overall effectiveness.